1. Field of the Invention
The present invention relates to a wet electrophotographic printer using a high density liquid developer, and more particularly, to a wet electrophotographic printer having a developing unit constructed so that an inlet of a nip between a developing roller and a metering roller communicates with a developer storing chamber to uniformly supply an appropriate high density liquid developer on a photosensitive body such as a photosensitive drum forming a visible developer image during a development.
2. Description of the Related Art
In general, an electrophotographic printer obtains a desired image by forming an electrostatic latent image on a photosensitive medium or photosensitive body such as a photosensitive belt or a photosensitive drum, developing the electrostatic latent image by using developers having predetermined colors, and transferring the developed electrostatic latent image to a paper. Electrophotographic printers are divided into wet type and dry type electrophotographic printers according to kinds of developers used therewith. The wet electrophotographic printer uses a liquid developer obtained by mixing a volatile liquid carrier with a powder type toner.
Since the wet electrophotographic printer using the liquid developer uses toner particles having a grain size less than about 0.5 to 5 μm, a higher quality image is obtainable for the wet electrophotographic printer than that of the dry electrophotographic printer using only the powder type toner in the development of the electrostatic latent image, and further, the wet electrophotographic printer prevents damage due to harmful toner dust. Therefore, the wet electrophotographic printer has gradually become popular to use.
However, the wet electrophotographic printer generally obtains an appropriate image density by using a low density liquid developer, which is below 3% solid. Accordingly, a complicated developer delivery system to sufficiently supply the liquid developer to developing regions of developing units and to collect the liquid developer is required to obtain the appropriate image density, thereby increasing a size of the developing unit and complicating the wet electrophotographic printer.
Further, an apparatus to control a density of the liquid developer is necessary to obtain the appropriate image density when replenishing the liquid developer due to variations of toner grains after the development.
Thus, the developer delivery system needs to be removed or to be simplified so as to prevent the wet electrophotographic printer from being oversized or being complicated. Accordingly, the wet electrophotographic printer using a high density liquid developer, which is over 3% solid, has been popularly used rather than a low density liquid developer, which is less than 3% solid.
FIG. 1 is a schematic view illustrating a general wet electrophotographic printer 1 using a high density liquid developer.
The wet electrophotographic printer 1 includes a photosensitive body 9 such as an organic photoconductive (OPC) drum, a laser scanning unit 11, a charging roller 12, a developing unit 13, a transfer belt 10 moving in a form of a caterpillar, primary and secondary rollers 21 and 22 rotating the transfer belt 10 in the form of the caterpillar, a first transfer roller 8 transferring a developer image to the transfer belt 10, a second transfer roller 23 transferring the developer image to a paper P, a fixing roller 25 fixing the developer image, and a cleaning blade 51 removing a residual developer image remaining on the transfer belt 10. The above-described elements of the wet electrophotographic printer sequentially perform an image formation process including a charging operation, an exposure operation, a development operation, a transfer operation and a fixation operation by mutual operations, to form a desired image on the paper P.
For color printing, the wet electrophotographic printer 1 includes four laser scanning units 11, and four developing units 13 each individually containing a high density liquid developer 48, each of which is of 3 to 40% solid and is of a different color, namely, the four individual high density liquid developers being, respectively, black, yellow, cyan and magenta in a color thereof.
Each of the developing units 13 includes a developing chamber 6 storing the high density liquid developer 48, a developing roller 7 positioned below the photosensitive body 9, a deposit roller 14 installed below the developing roller 7 to transmit an electric charge to the liquid developer 48 to form a charged developer layer on the developing roller 7, a metering roller 15 transmitting a predetermined voltage to the charged developer layer formed on the developing roller 7 by the deposit roller 14 to transfer a large volume of toner to the developing roller 7, and restricting the charged developer layer to have a predetermined amount or density (% solid) of the toner to supply into a nip between the developing roller 7 and the photosensitive body 9, and a cleaning roller 16 cleaning the developing roller 7.
The deposit roller 14 and the metering roller 15 serve to supply the predetermined toner amount or density of the charged developer layer into the nip between the developing roller 7 and the photosensitive body 9, regardless of the amount or the density of the solid, which is in a range of about 3 to 40% solid of the liquid developer 48.
To supply the liquid developer 48 to the developing chamber 6, disposed below each of the developing units 13 is a developer supply unit 28, 30.
Each of the developer supply units 28, 30 comprises a developer cartridge 28 connected to an inlet and an outlet formed, respectively, at a bottom and a side of the developing chamber 6 through connecting tubes 29 and serving as a developer storing chamber, and a pump 30 installed in each of the connecting tubes 29 positioned between the developer cartridge 28 and the inlet of the developing chamber 6.
Each of the developer cartridges 28 is coupled to a respective connecting tube 29 by a coupler (not shown) to facilitate during and exchange of the cartridge an attachment and a detachment thereof.
Operation of the wet electrophotographic printer 1 will now be explained.
According to a print command, an electric charge layer, namely an electrostatic latent image corresponding to an image to be printed is formed on the photosensitive body 9 by the charging roller 12 and the laser scanning unit 11 and then, a developer layer having a predetermined amount of toner formed on the developer roller 7 from the liquid developer 48 of the developing chamber 6 by the deposit roller 14 and the metering roller 15 is transferred to the electrostatic latent image, thereby forming a developer image.
Further, the liquid developer 48 is formed as a charged developer layer on the developing roller 7 due to an electric charge from the deposit roller 14, and formed as the developer layer having the predetermined amount of toner on the developing roller 7 by a predetermined voltage from the metering roller 15.
Further, the pump 30 supplies the liquid developer 48 into the developing chamber 6 from the developer cartridge 28 so that the developing chamber 6 overflows with the liquid developer 48 and the liquid developer 48, which is above a predetermined level, is withdrawn into the connecting tube 29 connected to the outlet of the developing chamber 6.
Accordingly, the liquid developer 48 is always maintained at the predetermined level in the developing chamber 6.
Thus, the developer image formed on the photosensitive body 9 by the developing unit 13 is transferred from the photosensitive body 9 to the transfer belt 10 by a voltage and a pressure of the first transfer roller 8 positioned inside the transfer belt 10. Since the transfer belt 10 rotates by the primary and secondary rollers 21 and 22, the image transferred to the transfer belt 10 is moved to the second transfer roller 23 and is subsequently transferred to the paper P by a voltage and a pressure of the second transfer roller 23.
The developer image transferred to the paper P is fixed to the paper P by the fixing roller 25 and a fixing backup roller 26, thereby forming the desired image.
After the developer image is transferred from the transfer belt 10 to the paper P, the transfer belt 10 continuously rotates by the primary and secondary rollers 21 and 22 and moves to the cleaning blade 51 installed to contact an image formation surface of the transfer belt 10 at a side of the primary roller 21. Further, remnants of the developer not transferred to the paper P (generally 90 to 98% of the developer is transferred to the paper P) are removed from the image formation surface of the transfer belt 10 by the cleaning blade 51 for succeeding image printing, and are collected in a used developer storing unit 52.
The transfer belt 10, from which the remnants of the developer are removed, repeats the aforementioned procedure to perform a subsequent electrostatic latent image formation and a subsequent development through the photosensitive body 9, the laser scanning unit 11 and the developing unit 13.
However, in the conventional wet electrophotographic printer 1, since the developer cartridge 28 of the developer supply unit 28, 30 is connected to the developing chamber 6 through the connecting tubes 29 to supply the liquid developer 48 thereto, the connecting tubes 29 may be clogged if the liquid developer 48 dries in the connecting tubes 29 and adheres thereto.
When the connecting tubes 29 are clogged as described above, the supply of the liquid developer 48 to the developing unit 13 is inefficient, thereby a quality of the developer image deteriorates. At worst, the developing unit 13 directly connected to the connecting tubes 29 is impossible to operate. In this case, all component parts including the developing unit 13 should be replaced by new component parts.
Further, to stably withdraw the liquid developer 48 overflowing from the developing chamber 6 into the developer cartridge 28 through the connecting tubes 29, the developer supply unit 28, 30 should be designed so that a free fall distance of the liquid developer 48 is maintained above a given range.
Further, since the developer cartridge 28 is attached and detached to the connecting tubes 29 by the coupler, exchanging of the developer cartridge 28 is troublesome, and if careless, the liquid developer 48 can flow out from the developer cartridge 28.
Since the pump 30 is relatively high-priced and used to supply the liquid developer 48 to the developing chamber 6, the manufacturing cost is increased.
Since each of the component parts, such as the developer cartridge 28, the developing unit 13 and the photosensitive body 9, which are disposed adjacent to each other, is constructed to be separately replaced, it is troublesome that the component parts are separately assembled and disassembled when problems related with the assembly as a whole occurs or that the component parts having almost identical life span are simultaneously replaced occurs.
To improve these problems, a wet electrophotographic printer 1′ having a single body developing unit 13′ in which a developing chamber and a developer cartridge are horizontally disposed and unified in a body so that a pump to supply a liquid developer is not required and replacement of individual component parts is not necessary as shown in FIG. 2.
In the wet electrophotographic printer 1′, a transfer belt 10′ is vertically disposed by primary and secondary rollers 21′ and 22′ installed in a longitudinally spaced-apart relation with each other to facilitate a disposal of remnants of developer remaining on the transfer belt 10′ after transferring a developer image to paper P and to prevent the remnants of the developer from moving to opposite ends of the primary roller 21′ and thereby contaminating the primary roller 21′, the transfer belt 10′ and other components of the wet electrophotographic printer 1′.
The wet electrophotographic printer 1′ includes a first transfer roller 8′ transferring a developer image to the transfer belt 10′, a second transfer roller 23′ transferring the developer image to a paper P, a fixing roller 25′ fixing the developer image, and a cleaning blade 51′ removing a residual developer image remaining on the transfer belt 10′. Further, remnants of the developer not transferred to the paper P are removed from the image formation surface of the transfer belt 10′ by the cleaning blade 51′ for succeeding image printing, and are collected in a used developer storing unit 52′.
The developer image transferred to the paper P is fixed to the paper P by the fixing roller 25′ and a fixing backup roller 26′, thereby forming the desired image.
To form a developer image of four colors, namely black, yellow, cyan and magenta, four image forming parts are installed at a side of the transfer belt 10′. Each of the image forming parts comprises a laser scanning unit 11′ horizontally disposed at a side of a photosensitive body 9′ to form an electrostatic latent image corresponding to an image to be printed according to an image signal, and the single body developing unit 13′ horizontally disposed below the photosensitive body 9′ to transfer a liquid developer 48′ to the electrostatic latent image formed on the photosensitive body 9′ to form the developer image.
As shown in FIG. 3, disposed about the photosensitive body 9′ are a discharging lamp 17, a corona charger 18, and a photosensitive body cleaner 19 having a cleaning blade 20 to clean the used remnants of the developer remaining on the photosensitive body 9′ after transferring the developer image from the photosensitive body 9′ to the transfer belt 10′, and a used developer reservoir 23.
The single body developing unit 13′ horizontally disposed below the photosensitive body 9′ comprises a developing chamber 6′ having a developing roller 7′, a deposit roller 14′, a metering roller 15′, and a cleaning roller 16′ installed therein; a developer storing chamber 28′ formed to horizontally extend and serving as a developer cartridge storing the liquid developer 48′; and a sponge roller 32 disposed to contact with a lower portion of partition 31 to divide the developing chamber 6′ and the developer storing chamber 28′ and to supply the liquid developer 48′ from the developer storing chamber 28′ to the developing chamber 6′.
When the sponge roller 32 rotates and is in compressive contact with the lower portion of the partition 31, the liquid developer 48′ absorbed by the sponge roller 32 in the developer storing chamber 28′ is moved into the developing chamber 6′, and then is discharged into the developing chamber 6′, so that the liquid developer 48′ is supplied from the developer storing chamber 28′ to the developing chamber 6′.
Thus, when the liquid developer 48′ is excessively supplied into the developing chamber 6′ by a rotation of the sponge roller 32, the liquid developer 48′ overflows a top end portion of the partition 31 and is withdrawn back into the developer storing chamber 28′.
In the wet electrophotographic printer 1′, since the single body developing unit 13′ has a structure in which the developing chamber 6′ and the developer storing chamber 28′ are unified in a body thereof, component parts are not separately assembled and disassembled in a replacement thereof, compared with the conventional wet electrophotographic printer 1. However, the wet electrophotographic printer 1′ presents a problem that the liquid developer 48′ having a high density stagnates in the developer storing chamber 28′ to form sediments of the toner contained therein on a bottom of the developer storing chamber 28′, since the developer storing chamber 28′ is formed to be horizontally and widely extended.
When the toner precipitates on the bottom of the developer storing chamber 28′, the density of the liquid developer 48′, which is formed as the charged developer layer on the developing roller 7′ due to an electric charge from the deposit roller 14′ in a vicinity of the deposit roller 14′ after moving from the developer storing chamber 28′ to the developing chamber 6′, is uneven according to a degree of precipitation of the toner, thereby a quality of the developer image formed during a development varies.
Therefore, to prevent the precipitation problem, an agitator needs to agitate the liquid developer 48′ in the developer storing chamber 28′, thereby manufacturing costs are increased.
Since the developer storing chamber 28′ is constructed to be horizontally and widely extended, a developer level of the liquid developer 48′ in the developer storing chamber 28′ may be lower than a top height of the sponge roller 32 even though the liquid developer 48′ is only slightly consumed.
In this case, when rotated, the sponge roller 32 absorbs air in the developer storing chamber 28′ and moves the absorbed air into the developing chamber 6′. Accordingly, when the sponge roller 32 is in compressive contact with the lower portion of the partition 31, the air absorbed by the sponge roller 32 in the developer storing chamber 28′ is discharged as air bubbles into the developing chamber 6′.
Thus, when the air bubbles form in the developing chamber 6′, the charged developer layer, which forms on the developing roller 7′ by the electric charge from the deposit roller 14′ in the vicinity of the deposit roller 14′, is uneven due to the air bubbles, thereby the quality of the developer image formed during the development varies.
Also, in the conventional wet electrophotographic printer 1′, since the metering roller 15′ is disposed in the developing chamber 6′, the liquid developer having a low density generated when the liquid developer 48′ is formed as the predetermined toner amount of developer layer on the developing roller 7 by a compression of the metering roller 15, may flow down from the metering roller 15 and may be directly supplied to the deposit roller 14′.
Thus, when the liquid developer of the low density is supplied to the deposit roller 14′, the density of the charged developer layer, which forms on the developing roller 7′ by the electric charge from the deposit roller 14′, is lower, thereby resulting in a problem that a density of the developed image is lower.